Abstract: A high-purity transparent silica glass containing Fe, Na and K impurities each in an amount of 0.01-0.3 ppm, and an OH group in an amount of 0-3 ppm; among the Fe impurities, the content of metallic Fe having a valency of +0 being not larger than 0.1 ppm. This transparent silica glass exhibits, even when it is maintained at 900-1,400.degree. C. for at least 20 hours, an extinction coefficient of not larger than 0.009 at a wavelength of 400 nm, and does not become colored as visually examined. The silica glass is made by a process wherein powdery silica filled in a mold cavity is melted at 1,700.degree. C. or higher, characterized in that the melting is conducted in a graphite mold having a porous high-purity graphite layer provided on the mold inner surface so that the filled silica is not contacted with the mold; said porous layer having a bulk density of 0.1-1.5 g/cm.sup.3, and the content of each of Fe, Na and K impurities in the porous layer being not larger than 1 ppm.

Abstract: An achromatic lens for ultraviolet rays constituted by (A) high-purity silica glass having a purity of 99.9% or more, or fluorine-containing, high-purity silica glass having a purity of 99.9% or more; and (B) silica glass containing germanium dioxide or silica glass containing germanium dioxide and boron oxide.

Abstract: High-hardness silica glass having transparency substantially on the same level as silica glass and a Vickers hardness of 12-20 GPa is produced by covering silica glass with powder and treating the silica glass at a temperature of 1400.degree.-2500.degree. C. and a pressure of 10-300 MPa in an atmosphere of an inert gas or a nitrogen gas.

Abstract: A method of producing a uniform silica glass block comprising using silica powder as a raw material, and treating it at high temperature and pressure by a hot press and/or a hot isostatic press in vacuum or in an inert gas atmosphere, preferably with a capsule of silica glass or a high-melting point metal. By conducting calcination in fluorine, chlorine or their compound gas and then in oxygen, the OH group content of the silica glass can be reduced dramatically.